His twin sister and little sister were playing in a room in the trailer in rural Knox, New York.
Kenneth White, five, was with Tiffany VanAlstyne, 19, their babysitter.
On Thursday, the babysitter, who is also the children's cousin, allegedly bashed the boy's head, strangled him and threw his body over a guardrail into a ditch.
She covered the ditch with snow, the Mail Online reported. She then concocted a story that Kenneth was kidnapped by two masked men.
Police went on a wild goose chase across Alany County to look for them.
Kenneth, Cheyenne and Christine were being cared for by their aunt Brenda VanAlstyne, the mother of Tiffany, after their parents lost custody of the children.
They lived with Mrs VanAlstyne for more than a year and she had been granted legal custody about nine months ago.
The mother of the children lives 40km away in Amsterdam, New York. Their father lives in Massachusetts.
It was not known why they lost custody of the children.
Police said that after a doctor's appointment on Thursday, Mrs Brenda VanAlstyne dropped off the three White children at her home and left Tiffany to babysit them.
Police believe she killed Kenneth just minutes after her mother left.
"I'm angry that we had a five-year-old killed and basically thrown away like a piece of trash. It's sinful and it's sad," Sheriff Craig Apple, who was visibly shaken by the horrific murder, said.
The daughter tried to cover her tracks by telling her mother that two masked men broke into the home, and held her down while they kidnapped her defenceless cousin and fled in a pickup truck, the New York Post reported.
Her mother then called the police, who began a search.
US Marshals and FBI, along with local police officers, joined in the hunt. But it was called off a few hours later when a police dog led officers to Kenneth's body, buried near the family's home.
After police began questioning Tiffany, her story began to unravel.
She was arrested and charged with second-degree murder on Friday.
Mrs Brenda VanAlstyne told the New York Post that she had no idea what happened to Kenneth.
She painted a picture of her daughter as a troubled teen who suffered from bipolar disorder but who had loved her young cousin.
She said her daughter was taking medication for the disorder.
"(Tiffany) was always taking care of him, buying gifts for him and she loved him so much," she told the Albany Times Union. "And we loved him, too."
Authorities described conditions inside the red-and-white striped trailer as dangerous and are working to make sure Kenneth's two sisters will not go back there.
Sheriff Apple said the horrific Christmas-time murder has rattled even veteran detectives on his police force.
His eyes welled with tears at a press conference as he described how he was going to go home to hug his three sons.
If convicted, Tiffany faces 15 years to life in prison.
This article was first published on December 21, 2014.
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